1. Technical Field
The present invention relates to a hydrogen ion-sensitive field effect transistor (FET) and a manufacturing method thereof. More particularly, the present invention relates to a hydrogen ion-sensitive FET applicable to a biosensor for measuring pH values and compatible with a transistor manufacturing process, and a method for manufacturing the hydrogen ion-sensitive FET.
2. Description of Related Art
As an analyzing device for measuring trace components, the so-called biosensor transforms information regarding the various measurement targets (e.g., glucose, blood sugar concentration, potassium ion concentration, cholesterol, etc.) in biomaterials into electronic or optical signals by use of biosensing elements (e.g., enzymes, antibodies, etc.) and physical principles so that subsequent analysis and processing are made easier.
However, during the measuring process, variations in pH values tend to affect the degree of ionization of various chemical substances in the biomaterials and alter the biomaterials or the measurement targets' activities. Therefore, a biosensor capable of measuring pH values is needed in order to provide accurate measurement of other bio-information items.
FIG. 1 is a schematic cross-sectional view of an ion-sensitive FET in the prior art.
Currently, biosensors commonly used for measuring pH values are ion-sensitive FETs (ISFETs), which are electrochemical sensing devices proposed by Piet Bergveld in 1970. ISFETs are advantageous in that they are miniaturized and suitable for automated measurement.
Referring to FIG. 1, the ISFET is similar in structure to a complementary metal oxide semiconductor (CMOS) manufactured by an integrated circuit (IC) manufacturing process. The ISFET includes a semiconductor substrate 10, a drain area 11, a source area 12, an insulating layer 13, an ion sensing film 14, a metallic layer 15, epoxy resin 16, an external reference electrode 17, and a buffer solution 20.
The ISFET differs from a CMOS in that a metallic gate of the CMOS is replaced by the ion sensing film 14, the buffer solution 20, and the external reference electrode 17. The ISFET can be immersed in buffer solutions 20 of different pH values. When the ISFET makes contact with the buffer solutions 20, different variations in the electrical potential arise at the interface between the metallic gate (i.e., the ion sensing film 14) and the buffer solution 20, thereby changing the current. By virtue of this property and through subsequent conversion and processing of the electrical signal measured by the reference electrode 17, the pH values or the concentrations of other ions in the buffer solution 20 can be evaluated.
In the last thirty years, a lot of research related to ISFETs have been conducted worldwide, as seen in research conducted on reference electrodes and miniaturization thereof, studies on ion-sensitive field effect devices using fixed enzymes, and discussion on differential front-end detection circuitries. Furthermore, attempts have also been made to look for more suitable and more stable ion sensing films, such as those made of Si3N4, Al2O3, Ta2O5, SnO2, a-WO3, a-Si:H, AlN, PbTiO3, or the like. However, such research is still within the basic framework of the prior art described above.